Going back to the semantic movement, strictly speaking em and strong should be used to encapsulate text with stress emphasis (when spoken) [0] and strong importance [1] respectively, and these properties would not necessarily be represented by italic and bold fonts (though, by default, they are).
A better choice for bold text that you don't think qualifies as "strong importance" would probably be the <b> tag, which denotes a "span of text offset from its surrounding content without conveying any extra emphasis or importance".
Comments
Going back to the semantic movement, strictly speaking em and strong should be used to encapsulate text with stress emphasis (when spoken) [0] and strong importance [1] respectively, and these properties would not necessarily be represented by italic and bold fonts (though, by default, they are).
A better choice for bold text that you don't think qualifies as "strong importance" would probably be the <b> tag, which denotes a "span of text offset from its surrounding content without conveying any extra emphasis or importance".
[0] http://dev.w3.org/html5/markup/em.html [1] http://dev.w3.org/html5/markup/strong.html [2] http://dev.w3.org/html5/markup/b.html